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Title Synthesis and crystal structure of K2NiF4-type novel Gd1+xCa1-xAlO4-xNx oxynitrides
Author(s) Masubuchi, Yuji; Hata, Tomoyuki; Motohashi, Teruki; Kikkawa, Shinichi
Citation Journal of alloys and compounds, 582, 823-826https://doi.org/10.1016/j.jallcom.2013.08.140
Issue Date 2013-01-05
Doc URL http://hdl.handle.net/2115/54928
Type article (author version)
File Information JALCOM2014-582-823-826.pdf
Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP
1
Synthesis and crystal structure of K2NiF4-type novel
Gd1+xCa1-xAlO4-xNx oxynitrides
Yuji Masubuchi*, Tomoyuki Hata, Teruki Motohashi, and Shinichi Kikkawa
Affiliation
Faculty of Engineering, Hokkaido University, N13 W8, Kita-ku, Sapporo,
Hokkaido 060-8628, Japan
*Corresponding author: Y. Masubuchi; Address: Faculty of Engineering,
Hokkaido University, N13 W8, Kita-ku, Sapporo 060-8628, Japan; Tel/Fax:
+81-(0)11-706-6742/6740; E-mail: yuji-mas@eng.hokudai.ac.jp
2
Abstract
Novel gadolinium calcium aluminum oxynitrides, Gd1+xCa1-xAlO4-xNx,
were prepared in x = 0.15-0.25 by the solid state reaction of a nitrogen-rich
mixture with AlN as an aluminum source; the mixture was sintered twice at 1500
C for 5 h under 0.5 MPa of nitrogen gas. Shift in the optical absorption edge
was observed in their diffuse reflectance spectra from 4.4 eV for the oxide (x = 0)
to 2.9 eV for the oxynitride at x = 0.2. The crystal structure of Gd1.2Ca0.8AlO3.8N0.2
at x = 0.2 was refined using a synchrotron x-ray diffraction data as a layered
K2NiF4-type structure with the I4mm space group. Longer Al-O/N bond lengths in
the oxynitride than those in GdCaAlO4 suggest that the nitride ions are in the
apical site of aluminum polyhedron, similar to those in Nd2AlO3N.
KEYWORDS: A. inorganic materials, A. rare earth alloys and compounds, B.
solid state reactions, C. crystal structure, D. synchrotron radiation, D. X-ray
diffraction
3
1. Introduction
Multinary oxynitride compounds are attracting much attention in
applications such as white light emitting diodes (LEDs) [1,2], visible light driven
photocatalysts [3,4], inorganic pigments [5,6] and dielectric materials [7-12]. The
optical properties of the oxynitrides in the UV-vis range have been explained by
the coexistence of nitride and oxide anions together because of the more
covalent nature in nitride ion [13]. Much research on phosphor materials for LED
applications has been conducted on silicon oxynitride and alumino-silicon
oxynitrides as host materials [14-16]. However, study on (oxy)nitrides of
especially aluminum is limited to systems such as AlN:Eu, spinel-type
AlON:Mg,Mn, and BaAl11O16N:Eu [17-19]. Magnetoplumbite-type aluminum
oxynitride doped with Eu was reported to be a phosphor material to have
emission in multiple wavelength [20]. Neutron diffraction study showed that the
emission site split because they have a slightly different coordination in the
presence of both nitride and oxide ions together [21].
RE2AlO3N oxynitride (RE = La, Nd, Sm) with K2NiF4-type structure (n = 1
in Ruddlesden-Popper phase, An+1BnX3n+1) has been prepared by firing mixtures
of RE2O3 and AlN in a small sealed nickel tube at a high temperature (1350 C)
4
under N2 flow [22]. Crystal structure refinement of Nd2AlO3N using its neutron
diffraction data showed an ordering of nitride and oxide ions in the reduced
symmetry from I4/mmm in K2NiF4 to I4mm in Nd2AlO3N [23]. The nitride ion is
located at an apical site of AlO5N octahedron. There are two kinds of rare earth
sites; Nd1 coordinated with O8N and Nd2 coordinated with O5N4, as shown in
Fig. 1. Sr2TaO3N crystallizes in K2NiF4-type structure with I4/mmm space group.
Its nitride ions are located at equatorial sites of TaO4N2 octahedron, and there is
only one crystallographic Sr site, (4e) [24]. Several kinds of aluminum oxides
AEREAlO4 (AE = Ca, Sr, RE = La, Nd, Sm, Gd) have also been reported to
crystallize in I4/mmm space group [25-27].
The oxynitride was found out in a preparation using a nickel tube. An
alternative preparation method, such as the carbon reduction nitridation method,
has been applied to prepare Nd2AlO3N and Sm2AlO3N [28]. La2AlO3N and
Gd2AlO3N can be utilized as a multi-color emitting phosphor host material when
they will be doped with divalent Eu. They have two crystallographic rare earth
sites induced by O/N ordering, as mentioned above for Nd2AlO3N.
Photoluminescence property of divalent Eu can be controlled by changing its
coordination environment such as coordination number and O/N ratio.
5
In our preliminary study, La2AlO3N has been tried to obtain in solid state
reaction but there was no appearance of the oxynitride in the products prepared
using a similar method described in this paper. Gd2AlO3N has not yet been
obtained in the carbon reduction nitridation [28], because the size of the Gd3+ ion
is not compatible with the K2NiF4-type structure. However, the GdCaAlO4 phase
is successfully prepared by the simple solid state reaction [29]. The ionic radius
of Gd3+ in 9 coordination is 0.124 nm, which is smaller than Nd3+ (0.130 nm) and
Sm3+ (0.127 nm); however, the mixing Gd3+ with Ca2+ (0.132 nm) will increase
their average size to stabilize K2NiF4 structure [30]. In this study, a series of
K2NiF4-type gadolinium calcium aluminum oxynitrides, Gd1+xCa1-xAlO4-xNx, were
prepared by co-substituting Ca2+ and O2- in GdCaAlO4 with Gd3+ and N3-
simultaneously by the solid state reaction. Their crystal structure was
investigated by high resolution synchrotron x-ray diffraction analysis.
2. Experimental
Gd2O3 (99.9%, Wako Pure Chemicals Co.), CaCO3 (99.9%, Wako Pure
Chemicals Co.), γ-Al2O3 (99.95%, Kojundo Chem. Lab. Co.), and AlN (Grade H,
Tokuyama Co.) were used as starting materials. Gd2O3 was calcined at 1000 C
6
for 10 h before mixing with other powders. The powders were dry mixed in
stoichiometric ratios for Gd1+xCa1-xAlO4-xNx with x = 0-0.3 in a dry nitrogen
atmosphere to avoid hydrolysis of AlN with moisture. The mixed powders were
uniaxially pressed at 20 MPa to form 10 mm diameter disks, which were then
fired in a gas pressure furnace at 1500 C for 5 h under a nitrogen pressure of
0.5 MPa. The aluminum source was either AlN only or a mixture of AlN and
γ–Al2O3. For AlN only as the aluminum source, the cation composition, i.e.,
Gd:Ca:Al = 1+x:1-x:1, was maintained; however, the anionic composition was
variable during the reaction. A part of AlN was expected to be oxidized during
synthesis process. When AlN without γ–Al2O3 was used, the starting mixtures
were fired twice with an intermediate grinding and mixing step.
The crystalline phases were characterized by powder x-ray diffraction
(XRD; Ultima IV, Rigaku) with monochromatized Cu Kα radiation. XRD patterns
were collected over the 2θ range of 10-120 with a step size of 0.02. For
structural refinement of Gd1+xCa1-xAlO4-xNx, high resolution synchrotron powder
XRD experiments was performed at room temperature using a Debye-Scherrer
camera installed at beamline BL02B2 of the Japan Synchrotron Radiation
Institute (SPring-8). The incident beam was monochromatized to 0.035459 nm.
7
The finely ground powder samples were put into a 0.2 mm glass capillary. The
Rietveld program RIETAN-FP [31] was used for crystal structure refinement. The
crystal structures were visualized using the VESTA program [32]. The nitrogen
content was measured with an oxygen/nitrogen analyzer (EMGA-620W, Horiba)
using Si3N4 as a reference. Diffuse reflectance spectra were measured using a
spectrophotometer (V-550, Jasco) in the range of 250-750 nm.
3. Results and discussion
3.1. Preparation of Gd1+xCa1-xAlO4-xNx oxynitrides
The products obtained from stoichiometric mixtures using both γ–Al2O3
and AlN as aluminum source were contaminated with Gd2O3. A single phase of
Gd1+xCa1-xAlO4-xNx with the K2NiF4-type structure was obtained only at x = 0, that
is GdCaAlO4. Simultaneous substitution of Ca2+ and O2- pair with Gd3+ and N3-
together led to the K2NiF4-type oxynitride products with a small impurity phase of
Gd2O3. The amount of the Gd2O3 impurity phase increased with an increase in x.
Diffraction lines from the (00l) planes of the K2NiF4-type structure shifted toward
a smaller diffraction angle with increasing x. Elongation of the c-axis has been
observed similarly from the CaNdAlO4 oxide (a = 0.36847(3) nm and c =
8
1.2124(2) nm) to Nd2AlO3N oxynitride (a = 0.37046(2) nm and c = 1.25301(13)
nm) [23,27]. The increase of the lattice parameter c suggests the formation of
Gd1+xCa1-xAlO4-xNx oxynitride with the K2NiF4-type structure. The appearance of
Gd2O3 impurity may indicate nitrogen deficiency in Gd1+xCa1-xAlO4-xNx due to
partial oxidation of AlN during the synthesis.
The K2NiF4-type Gd1+xCa1-xAlO4-xNx oxynitride obtained from starting
mixtures containing both γ-Al2O3 and AlN was contaminated with the Gd2O3
impurity phase loosing nitrogen during the reaction. Therefore, AlN only was
used as the aluminum source in the starting mixtures to improve the phase purity.
Single phase of Gd1+xCa1-xAlO4-xNx was obtained between x = 0.15 and 0.25 as
shown in Fig. 2. AlN was expected to be partially oxidized during the synthesis
process. The suitable O/N ratio was attained to form a pure oxynitride phase
between x = 0.15 and 0.25 in the reaction.
CaAl2O4 and Gd2O3 were observed as secondary phases at x = 0.1 and
0.30, respectively. The product obtained at x = 0.2 had an expanded c-axis (a =
0.3658(1) nm and c = 1.2067(5) nm) compared to that for GdCaAlO4 (a =
0.3658(2) nm and c = 1.1994(6) nm) at x = 0. The nitrogen content of the x = 0.2
oxynitride was 1.2(1) wt%, which is comparable with the theoretical nitrogen
9
content of 0.9 wt% within 3σ. The x = 0.20 oxynitride was expected to have an
O/N ratio of 3.8/0.2 to maintain the charge neutrality. The lattice expansion may
be due to partial substitution of O2- (r = 0.124 nm, c.n. = 4) by N3- (r = 0.134 nm,
c.n. = 4), although Gd3+ (r = 0.124 nm, c.n. = 9) is smaller than Ca2+ (r = 0.132
nm, c.n. = 9) [30]. Anisotropic expansion along the c-axis implies the ordering of
nitride ions in the apical sites of AlO5N octahedron, similarly to the crystal
structure reported for the Nd2AlO3N oxynitride with fully ordered nitride ions on
the apical site (2a), as shown in Fig. 1(a) [23].
The color of the resultant oxynitrides was yellow, while GdCaAlO4 was
white. The absorption edges were at ca. 4.46 eV and 2.94 eV for the oxide (x =
0) and oxynitride (x = 0.2), respectively, as shown in Fig. 3. The color change
occurred with the nitrogen incorporation to form a new valence band at higher
energy position than O(2p) orbital level, as reported for many oxynitride
pigments [5,6,13]. The edge positions of the oxynitrides obtained at x = 0.15 ~
0.25 slightly shifted toward longer wavelength along with the nitrogen content, x.
The shift in edge positions indicated a decrease in band gap of
Gd1+xCa1-xAlO4-xNx, implying an elevated valence band level as increasing in the
nitrogen content. Table 1 summarizes the optical band gap energy and lattice
10
parameters of the oxynitrides. Ordering of the nitride ions in the apical site of Al
octahedron induces two kinds of Gd/Ca sites coordinated with N-rich or N-poor
anions as shown in Fig. 1(a). Absorption edge positions can be attributed to the
N-rich Gd/Ca polyhedron, because of the higher energy level of the nitrogen 2p
orbital than that of oxygen. The decrease in band gap energy might be
interpreted as nitrogen enrichment in the former Gd/Ca polyhedron. Crystal
structure of the oxynitride at x = 0.20 is discussed in the next section.
3.2. Crystal structure refinement at the Gd1.2Ca0.8AlO3.8N0.2 oxynitride
The crystal structure of Gd1.2Ca0.8AlO3.8N0.2 was refined using the high
resolution synchrotron XRD pattern in the K2NiF4-type structure with the I4mm
space group, starting from the reported crystallographic parameters for
Nd2AlO3N [23]. The O/N ratios for three anionic sites were fixed at 0.95/0.05,
because of their mutually similar x-ray scattering factors. Neutron diffraction has
been generally used for analysis of the oxygen and nitrogen distribution in
oxynitrides; however, it is not useful for the present products, because the Gd
atom has a large neutron absorption cross section. The finally refined
parameters from the Rietveld method are summarized in Table 2. Observed,
11
calculated, and difference synchrotron XRD profiles for Gd1.2Ca0.8AlO3.8N0.2 are
shown in Fig. 4. The refined occupancies for Gd/Ca sites indicate a chemical
composition of Gd1.22(1)Ca0.78(1)AlO3.8N0.2, which is in agreement with the nominal
starting composition of Gd1.2Ca0.8AlO3.8N0.2. The bond lengths and the atomic
arrangements around Al and Gd/Ca sites are summarized in Table 3 and shown
in Fig. 5(a), respectively. The crystal structure of GdCaAlO4 reported by L.
Vasylechko, et al., [26] using a powder XRD pattern is also shown in Fig. 5(b).
The oxynitride has c-axis much longer than the reported value for GdCaAlO4
(1.19787 nm), while a-axis is almost similar between the oxynitride and oxide.
The elongated c-axis is attributed to longer Al-O/N2 bond length of 0.2160(18)
nm in the oxynitride than the corresponding distance of 0.2027 nm for Al-O in the
oxide. Similarly elongated bond lengths have been observed for Al-N bond
length in Nd2AlO3N compared with NdCaAlO4. The nitride ions are located at the
apical sites of AlO5N octahedron in Nd2AlO3N [23]. The bond length of Al-N was
reported to be 0.1893 nm for c.n. = 4 in AlN, which is longer than that of Al-O
(0.1710 nm for c.n. = 4 in γ-Al2O3) [33,34]. The longer bond lengths of Al-O/N2
indicate the preferential occupation of nitride ions on O/N2 sites in the present
Gd1.2Ca0.78AlO3.8N0.2, as reported in Nd2AlO3N [20]. The preferential occupation
12
of nitride ion might elongate the chemical bond between Gd/Ca and O/N2. On
the other hand, the refined bond length is 0.2617(2) nm for Gd/Ca1-O/N2 which
is comparable to the reported value of 0.2609 nm in the corresponding oxide.
Substitution of O2- with N3- can increase the bond length, while smaller Gd3+
cation also replaces a part of bigger Ca2+ cation in the oxynitride. Therefore the
bond length between Gd/Ca and O/N does not change significantly from the
oxide to the oxynitride. BVS calculation was performed in the oxynitride. The
value is +3.06 for the Al ion and is larger than theoretical value of +3, when a
random distribution of O/N ions on the anion sites is assumed. The BVS value is
improved to be +3.01 in the ordering of the nitride ions only in O/N2 site,
supporting the preferential occupation of nitride ions in O/N2 site. Structure
refinement in I4/mmm space group showed much less agreement with Rwp value
of 5.6 %. The refined bond length of Al-O/N2 was shorter than that of Al-O in the
oxide, although the refined Gd/Ca1-O/N2 bond length was longer than that in the
oxide. Both the poor fitting of the oxynitride and the decrease in the Al-O/N2
bond length of Al(O,N)6 octahedron exclude the I4/mmm space group for the
structural refinement of the Gd1.2Ca0.8AlO3.8N0.2 oxynitride. The present
oxynitride crystallizes in I4mm space group having non-equivalent Gd/Ca1 and
13
Gd/Ca2 sites. Preparation and photoluminescence property of Eu doped
Gd1+xCa1-xAlO4-xNx are currently being investigated.
4. Conclusions
Novel gadolinium calcium aluminum oxynitrides, Gd1+xCa1-xAlO4-xNx (x =
0.15-0.25), were prepared by the solid state reaction from a nitrogen-rich starting
composition. Diffuse reflectance spectra showed the shift in absorption edge
from oxide (4.46 eV) to oxynitride at x = 0.2 (2.94 eV) due to the nitrogen
incorporation. The crystal structure refinement of the Gd1.2Ca0.8Al3.8N0.2
oxynitride using synchrotron XRD showed the elongation of the Al-O/N bond
length in aluminum octahedron similar to the Nd2AlO3N, indicating that the nitride
ions are preferentially in the apical site of the aluminum octahedron in the
oxynitride. The preferential occupation of nitride ion on a specific anion site
forms two distinct Gd/Ca sites making the oxynitride to future application for a
novel muti-color emitting phosphor host material for divalent Eu doping.
14
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18
Table 1
Band gap energies and lattice parameters for Gd1+xCa1-xAlO4-xNx.
x composition band gap / eV lattice parameters
a
a / nm c / nm
0.0 GdCaAlO4 4.46 0.3658(2) 1.1994(6)
0.15 Gd1.15Ca0.85AlO3.85N0.15 2.96 0.3659(3) 1.2059(9)
0.20 Gd1.2Ca0.8AlO3.8N0.2 2.94 0.3658(1) 1.2067(5)
0.25 Gd1.25Ca0.75AlO3.75N0.25 2.91 0.3658(2) 1.2072(7)
alattice parameters were calculated from the laboratory XRD data.
19
Table 2
Refined structural parameters of Gd1.2Ca0.8AlO3.8N0.2 in K2NiF4-type structure.
Atom Site g x y z Biso / 10-2
nm2
Al 2a 1 0 0 0.0118(9) 0.56(3)
Gd/Ca1 2a 0.614/0.386(8) 0 0 0.3657(4) 0.33(6)
Gd/Ca2 2a 0.606/0.394(5) 0 0 0.6509(4) 0.35(6)
O/N1 4b
0.95/0.05a
0 1/2 0.0151(8)
0.38(6)b O/N2 2a 0 0 0.8329(8)
O/N3 2a 0 0 0.1689(7)
Rwp = 5.4 %, Rp = 4.1 %. S.G.: I4mm, a = 0.365850(1) nm, c = 1.206752(8) nm. aSite
occupations, g, for anion sites were fixed as 0.95O/0.05N. bIsotropic displacement
parameters, Biso, for anion sits were analyzed using a constrains: B(O/N1) = B(O/N2) =
B(O/N3).
20
Table 3
Selected bond lengths (nm) in Gd1.2Ca0.8AlO3.8N0.2.
Gd1.2Ca0.8AlO3.8N0.2 S.G.: I4mm
Al (2a) O/N1 (4b) 4 0.18297(2)
O/N2 (2a) 1 0.2160(18)
O/N3 (2a) 1 0.1900(19)
Gd/Ca1 (2a) O/N1 (4b) 4 0.2568(8)
O/N2 (2a) 4 0.2617(2)
O/N3 (2a) 1 0.2375(11)
Gd/Ca2 (2a) O/N1 (4b) 4 0.2456(8)
O/N2 (2a) 1 0.2197(11)
O/N3 (2a) 4 0.2596(2)
21
Figure Captions
Fig. 1 Crystal structures of K2NiF4-type oxynitrides: (a) Nd2AlO3N (I4mm) and (b)
Sr2TaO3N (I4/mmm).
Fig. 2 XRD patterns of products obtained from starting mixtures with AlN only as
the aluminum source for Gd1+xCa1-xAlO4-xNx at x = (a) 0, (b) 0.05, (c) 0.1, (d) 0.15,
(e) 0.20, (f) 0.25, and (g) 0.30. The product obtained for x = 0 (GdCaAlO4, (a))
was prepared using γ-Al2O3 as the aluminum source. Diffraction lines marked
with diamonds, triangles and circles indicate the K2NiF4-type Gd1+xCa1-xAlO4-xNx,
Gd2O3, and CaAl2O4 phases, respectively.
Fig. 3 Diffuse reflectance spectra of Gd1+xCa1-xAlO4-xNx.
Fig. 4 Observed (+), calculated (solid line) and difference synchrotron XRD
profiles for K2NiF4-type Gd1.2Ca0.8AlO3.8N0.2. Vertical bars indicate the positions
of Bragg reflections.
Fig. 5 Atomic arrangement around Al and Gd/Ca sites in (a) Gd1.2Ca0.8AlO3.8N0.2
22
(I4mm) and (b) GdCaAlO4 (I4/mmm). The schematic structure of GdCaAlO4 was
drawn using the reported structural parameters [26].
23
Figure 1 Y. Masubuchi, et al.
Fig. 1 Crystal structures of K2NiF4-type oxynitrides: (a) Nd2AlO3N (I4mm) and (b)
Sr2TaO3N (I4/mmm).
24
Figure 2 Y. Masubuchi, et al.
Fig. 2 XRD patterns of products obtained from starting mixtures with AlN only as
the aluminum source for Gd1+xCa1-xAlO4-xNx at x = (a) 0, (b) 0.05, (c) 0.1, (d) 0.15,
(e) 0.20, (f) 0.25, and (g) 0.30. The product obtained for x = 0 (GdCaAlO4, (a))
was prepared using γ-Al2O3 as the aluminum source. Diffraction lines marked
with diamonds, triangles and circles indicate the K2NiF4-type Gd1+xCa1-xAlO4-xNx,
Gd2O3, and CaAl2O4 phases, respectively.
25
Figure 3 Y. Masubuchi, et al.
Fig. 3 Diffuse reflectance spectra of Gd1+xCa1-xAlO4-xNx.
26
Figure 4 Y. Masubuchi, et al.
Fig. 4 Observed (+), calculated (solid line) and difference synchrotron XRD
profiles for K2NiF4-type Gd1.2Ca0.8AlO3.8N0.2. Vertical bars indicate the positions
of Bragg reflections.
27
Figure 5 Y. Masubuchi, et al.
Fig. 5 Atomic arrangement around Al and Gd/Ca sites in (a) Gd1.2Ca0.8AlO3.8N0.2
(I4mm) and (b) GdCaAlO4 (I4/mmm). The schematic structure of GdCaAlO4 was
drawn using the reported structural parameters [26].
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